The diagnostics and monitoring of structures, such as that carried out in the structural health monitoring field, are often accomplished by employing arrays of piezoelectric sensing elements and/or actuators. However, such arrays are limited in several respects. As one example, electromagnetic interference, or crosstalk, often places a lower limit on the spacing between actuators and sensors.
To illustrate this concept, FIG. 1 shows the operation of a typical sensor and actuator used in structural health monitoring. Here, an actuator 10 is shown, which is often a piezoelectric transducer capable of changing its shape according to an applied voltage, so as to generate a stress wave, as shown. That is, when attached to a structure, the actuator 10 converts an electrical input signal 20 to a stress wave that propagates through a structure to a sensor 30 placed a distance d away, where it is detected and converted to an electrical output signal 40. However, the sensor 30 also picks up the electromagnetic interference from the input signal 20. Accordingly, the output signal 40 has a crosstalk portion 50 caused by interference from the input signal 20, as well as a stress wave portion 60 corresponding to the detected stress wave. Typically, the signal of interest is the stress wave portion 60, while the crosstalk portion 50 is regarded as a nuisance.
It is known that the amount by which the crosstalk portion 50 and stress wave portion 60 are separated is a function of the distance d between the actuator 10 and sensor 30. That is, as the distance d decreases, the crosstalk portion 50 and stress wave portion 60 move closer together. Conventionally, the minimum distance d that an actuator 10 and sensor 30 can be placed together is the point at which the crosstalk portion 50 and stress wave portion 60 begin to overlap:dmin=νwavetinput  (1)wheredmin=conventional minimum distanceνwave=velocity of generated stress wavetinput=time duration of actuator input signal
For optimal structural health monitoring, it is often desirable to position actuators closer to sensors than the distance dmin. However, doing so requires somehow dealing with overlap between the crosstalk portion 50 and stress wave portion 60, as the crosstalk portion 50 alters the stress wave portion 60, commonly resulting in invalid sensor readings. It is therefore desirable to develop methods for reducing crosstalk between actuators and sensors, so as to allow for more densely positioned actuators 10 and sensors 30, and more accurate/reliable structural health monitoring.